It's fine for them to exist, and probably better -- you can land incomplete things behind a feature flag and avoid some amount of rebase/merge hell.
But they should be marked experimental in that case, not optional. Optional flags ought to work together, or at least it should be considered a bug worth quickly fixing if they don't. Combining experimental flags otoh is Buyer Beware.
It's fine for them to exist, and probably better -- you can land incomplete things behind a feature flag and avoid some amount of rebase/merge hell.
But they should be marked experimental in that case, not optional. Optional flags ought to work together, or at least it should be considered a bug worth quickly fixing if they don't. Combining experimental flags otoh is Buyer Beware.
You’re assertively conflating two very different things (experimental flags and optional flags).